The present invention relates to an apparatus for friction stir welding, comprising a welding unit including a welding head, a worktable including a backing unit to support the workpieces to be welded together along their joint line, and clamping means to clamp the workpieces to the worktable during the welding operation.
The definition friction stir welding designates a welding method according to which the workpieces to be joined together are plasticized along their joint line by supply of frictional heat from a rotating welding probe, the rotational axis of which extends essentially at right angles to the surface of the workpiece, and said welding probe is mounted on a welding head, which displaces the welding probe along the joint between the workpieces while at the same time it is being pressed against the workpieces. As described in WO93/10935 and WO95/26254, the welding probe should be manufactured from a harder material than the workpieces, and during the welding operation the workpieces should be securely fixed relative to one another and to the worktable. In this respect this technique differs from that of conventional friction welding, according to which the frictional heat is generated by the relative motion of the workpieces as they are being pressed together, i.e. the frictional heat is generated only by the two components to be joined together. The configuration of the welding probe is conditional e.g. on the nature of the material to be welded and on the intended application.
The friction stir welding technique is particularly suitable for welding together aluminium profile sections for construction of large-size aluminium plating intended for example for hulls. The reason therefor is that this welding technique produces a welded joint the bottom face of which has a fine and smooth finish, with resulting few points of load concentration. In addition, the strength of the joint produced by this joining method is almost equal to that of the basic material. The considerable forces to which the workpieces are exposed during the very welding operation do, however, place considerable demands upon the stability of the worktable and on the ability of the clamping means to fix the workpieces in position.
In addition, this welding technique can only be used to join together workpieces that are separated by a very narrow air gap. Should the air gap exceed this critical value, which among other things depends on the thickness of the profile sections, a joint is produced having internal cavities and impaired strength. In consequence thereof, extruded aluminium profile sections that cannot be produced to exactly precise dimensions by existing manufacturing methods, sometimes require some pre-treatment to avoid too large gaps between the profile sections. This is particularly important in the case of the welding together of elongate profile sections, in which even small deformations may give rise to unacceptable deviations.
Considering that deformations in the profile sections may occur also during the friction stir welding operation proper, since the sections are exposed to the effects of heat, the dimensions of the end product may fail to be within the acceptable tolerance range, despite the pre-treatment of the profile sections. The deformations are smaller than in the case of MIG and TIG welding, since the temperature never reaches the fusing point.
Another consequence of the critical size of the air gap is that the workpieces must not move relative to one another once they have been fixed in this correct welding position. In other words, high demands are placed also on the ability of the clamping means to fix the positions of the workpieces. Considering that the welding probe must be in contact with both workpieces during the welding operation, high demands are placed also on the running accuracy of the welding head.
In order to meet the above criteria, a complex and consequently expensive welding apparatus is required.
In addition, in order to ensure a reasonable repayment period for an investment of this magnitude, another requirement is that the welding apparatus possess a high degree of availability, which means that the time required to adjust the welding apparatus to the configuration of the workpieces, the thickness of the material, the desired appearance of the intended joint, for example as a corner joint, etcetera, in principle should be negligible while at the same time the above-mentioned demands concerning the small air-gap size etcetera should be met. This is a considerable problem in workshops, where one wishes for example to weld together hollow profile sections during part of the working day and solid workpieces having identical or different material thickness during another part of the day, without having to resort to time-consuming re-setting operations when shifting from one mode of operation to another, while at the same time the production of a high-precision welded joint to be formed along the joint line of the workpieces must be guaranteed.
The object of the present invention therefore is to make it possible to weld together workpieces of various configurations by means of the above welding apparatus without requiring time-consuming re-settings.
This object is achieved in accordance with the teaching of the present invention by means of a welding apparatus of the kind defined in the introduction hereto and which is characterised in that the backing unit consists of at least two backing parts or of at least one backing part and one additional welding head, said backing parts being freely exchangeable, one for the other, at each welding operation as is also each backing part for an additional welding head, and vice versa.
Owing to the two-part design of the welding apparatus backing unit, consisting of at least two backing parts or of at least one backing part and one additional welding head, it becomes possible, during operation, to rapidly re-set the welding apparatus before each welding operation, from a welding mode according to which solid workpieces the material of which is of equal or different thickness are welded together, to a welding mode according to which workpieces in the form of hollow profile sections are welded together.
The backing parts and/or the additional welding head may be displaceable in a direction perpendicular to the joint line of the workpieces.
The welding unit and the clamping means may be displaceable in a direction perpendicular to the direction of movement of the welding unit relative to the backing unit in order to allow the joint line of the workpieces to be changed. At least one backing part may be formed with an even-level upper face or with a stepped upper face. One backing part may be a separate backing element, such as a bottom backing element, or else two backing parts may be formed by two backing pieces, such as bottom element pieces. In addition, the backing unit may consists of at least one separate backing element and at least two backing pieces.
It is likewise possible for the backing unit to consist of three different backing elements, one of which could in this case be formed with a plane upper face whereas the other two backing elements be formed with a stepped upper face, the steps of the latter backing elements being of different heights.
The welding apparatus may be fitted with a means for shifting the additional welding head from a position below the joint line of the workpieces to a position above said line. The additional welding head preferably is a friction stir welding head.
One of the friction stir welding heads could be arranged to alternatively serve as a milling head. This arrangement makes it possible to perform a milling operation prior to and/or following each separate welding operation, which ensures acceptable tolerance ranges in the discrete workpieces as well as in the finished product without the workpieces or product having to be passed through any external pre- or post-treatment unit. Owing to the milling step, the workpieces will be given such a degree of straightness that any inter-workpiece air gaps will not exceed the value at which a faulty joint is produced.